SUMMARY ? Cell Tool Segment A brain cell atlas is essential but not sufficient for understanding brain function. Armed with knowledge of the cell census and wiring diagram of the brain, neuroscientists further need to be able to systematically observe and manipulate the activity of defined cell types across many contexts. This requires reliable experimental access to comprehensive sets of neuron types. The overall goal of the Cell Tool Segment is to leverage molecular and anatomical information gained in the Molecular and Anatomical Research Segments to establish experimental access to (1) a comprehensive set of forebrain projection neuron types and to (2) rare and/or fragile populations of cells from the molecular whole-brain Atlas. In addition, we will generate reporter lines and viral vectors that enable Cre / Flp labeling based on the intersection or subtraction of two driver lines and/or anatomical (viral) labeling, enabling greater specificity in labeling through application of more complex identifying criteria. We aim to generate a restricted but highly selected set of knock-in driver lines that achieves maximal coverage of neuron types at high specificity. We will maximize coverage, specificity, and impact through combinatorial use of Cre and FlpE alleles, anterograde and retrograde viral vectors, and/or temporal regulation of recombinase activity to label neuron types based on intersecting criteria. Cell populations captured by novel markers and driver lines will feed back into the Molecular and Anatomical segments in an iterative process to refine cell type definition. In addition, these tools will allow the application of a full range of modern techniques to defined cell populations to achieve concerted in-depth studies, including physiological and functional studies during circuit operation and behavior. These genetic tools will not only further our Center's goal of assembling a single-cell atlas of the brain, but also provide the field with powerful, validated tools to enable systematic studies of brain network organization, function and development.